A charismatic personality can go a long way in patching over a film’s shortcomings. But it’s another problem when that charismatic personality is the sole point of a film.

Melissa McCarthy collaborated on the script for “Tammy” with her husband, Ben Falcone, who also directed the movie. So it is, more especially than any of McCarthy’s other projects (”Heat,” ‘’Identity Thief,” ‘’Bridesmaids”), a film that’s tailor-made to suit the actress’ comedic strengths. But when that chief strength is being super good at being obnoxious, a little goes a long way.

Tammy’s life starts out a complete disaster and gets worse from there. She’s a mess of a woman, heading to her fast-food job in a rusted green Toyota Corolla that’s older than the boombox in her passenger seat blasting the Outfield’s “Your Love.” She smashes into a deer on her way to work, further wrecks her already pretty wrecked car, arrives to work late and gets fired then comes home in the middle of a workday to find that her husband has been cheating on her with a neighbor.

She’s desperate to get out of town and try something new but lacks the funds and transportation. So she strikes a Faustian bargain with her hard-drinking, diabetic grandmother Pearl (Susan Sarandon), who’s also desperate for change -- if Pearl fronts the cash and her car, Tammy will drive her to Niagara Falls, a place Pearl has always wanted to visit.

So they set out on the road, Pearl with her granny bag loaded with beer, whiskey, Oxycodone and hard cash, itching to raise a little hell —which she does, in the backseat of her car in Louisville. Meanwhile, Tammy starts to connect with Pearl’s paramour’s son, Bobby (Mark Duplass), who’s at first somewhat repulsed by Tammy but warms to her unpredictability.

“My life’s boring,” he says, “and you’re a very not-boring person.”

And it’s a very not-boring film. In addition to McCarthy and Sarandon, it’s packed with hilarious women in substantial roles: Sandra Oh, Allison Janney, Toni Collette. The best is Kathy Bates, who plays a rich lesbian who owns a chain of pet-supply stores and drives an RV with puppy-printed seat covers, when she’s not lobbing Molotov cocktails and flaming spears while screaming, “Valhalla!”

They’re all fun yet digestible characters who strike a balance between humor and sincerity. But when it comes to McCarthy, the film gets so distracted with the need to score laughs that it does so at the expense of developing her character, and it’s a constant source of tonal whiplash.

The film ricochets between Tammy being an oblivious cartoon goblin and a textured, sympathetic human being who just wants to be loved. Perhaps if the film had catered a little less to McCarthy’s comedic gifts — the curse-word fugue states, the slapstick humor, the non sequiturs — the result would have felt more balanced and rewarding.

Yes, that would mean cutting some of the funniest bits, but sometimes you’ve got to kill your darlings. Is the film a slapstick comedy in which a CGI deer hoofs a woman in the chest and knocks her down, or is it a thoughtful comedy-drama about a woman at a midlife crossroads attempting to change her trajectory? “Tammy” tries to be both. The result is equal parts funny and heartfelt -- but also less than the sum of its parts.